Add two new cluster settings

The new variables are:

- a list of hidden OSes, that should not be displayed to the users in
  interactive selection (e.g. reinstall); however, if they are selected, they
  can be used
- a list of OSes that should be hidden and blocked from install-time selection

The filtering will apply at pure OS name level, not OS+variant level.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>